1. Field of the Invention
The invention relates to a control method of a display apparatus and, more particularly, to a control method suitable to display video information by using a display apparatus with a liquid crystal having a memory performance in a liquid crystal display apparatus in which scan signal lines and information signal lines are arranged in a matrix form and are driven by applying scan signals and information signals to those signal lines, respectively.
2. Related Background Art
Hitherto, as a computer terminal display apparatus, a CRT of the refresh scan type has mainly been used. A frame frequency of 60 Hz or higher is used in order to prevent flickering (of the screen). A non-interlacing method is used in order to improve visibility of a moving display (movement of a mouse, an icon, or the like) of information on the screen. Therefore, as display resolution rises, a high power is required and a drive control unit also increases in size and cost. For instance, in case of a television, an interlacing method is used for convenience of the display of a moving image and a drive control system, a field frequency is set to 60 Hz, and a frame frequency is set to 30 Hz.
In recent years, as a computer terminal display apparatus, attention has been paid to a flat panel display in consideration of the inconvenience such as large size and high power of the CRT mentioned above.
At present, as a flat panel display, there are several methods such as a high time-division method (STN) of twisted nematic liquid crystal, a black and white display method (NTN) as a modification of the STN method, a plasma display method, and the like. However, the flat panel display of each of the above methods uses the same image data transfer method as the CRT and also uses the non-interlacing method having a frame frequency of 60 Hz or more as a picture plane updating method. This is because each of those display panels does not have the memory performance on the display principle, so that a refresh cycle of the frame frequency of 60 Hz or higher is needed from a viewpoint of prevention of flickering. On the other hand, even in a method (TFT, MIN, TFD, or the like) in which a switching transistor or a non-linear element is formed in each pixel of the twisted nematic liquid crystal, the image information can be held within at most one frame. Therefore, a refresh cycle of 60 Hz or more is necessary in a manner similar to the above methods.
On the other hand, since a ferroelectric liquid crystal display apparatus has a feature such that the image information which has once been displayed can be held (memory performance), it is possible to display a fairly larger screen at a higher precision than those of the various kinds of display apparatuses mentioned above. However, since the frame frequency at which the apparatus can be driven is low, in order to cope with the display apparatus of the main-machine interface, a partial rewriting scanning (only the scan lines whose image information is changed are scanned (driven)) method using the memory performance is necessary. As such a partial rewriting scanning method, a method such as "the low frame frequency driving (multi interlace scan) +the partial rewriting scan" to display at a high resolution in a display apparatus having the memory performance has been realized as proposed in U.S. Pat. Nos. 5,058,994 and 5,091,723 by Mihara et al., and the like on the basis of the method proposed in, for example, U.S. Pat. No. 4,665,561 by Kanbe et al., or the like.
In the conventional control method of the ferroelectric liquid crystal display device, when there is a change in image information, the partial rewriting scan is executed, and when there is no change, the whole screen refresh scan by the multi interlace or the like is executed. Therefore, for example, even in a state in which the operator leaves his seat or the like and a key input or an operation by the mouse is not performed for a long time, the display panel continuously executes the refresh scan by the multi interlace or the like.
As mentioned above, as one of the major features, the ferroelectric liquid crystal display apparatus has the memory performance and, fundamentally, when there is no change in image information, the display state can be held without performing the refresh scan (driving). According to the conventional drive control method, however, even in a state such that the key input or the mouse operation is not executed for a long time, it is impossible to perform a control such as to keep the display content by stopping the scan (driving) of the display panel, that is, by setting the display panel into the memory state.